Pressurized water nuclear reactors include steam generators which ensure supply water heating and vaporization by the heat conveyed by the pressurized water used for cooling the core of the reactor.
Pressurized water reactors include on each of their primary branches a steam generator having a primary portion in which the pressurized water circulates for cooling the reactor and a secondary portion receiving the supply water, which is heated and gradually vaporized and flows out of the secondary portion of the steam generator as steam which is sent to the turbine associated with the nuclear reactor in order to ensure driving of an alternator for producing electric current.
Such steam generators include an external shell, called a pressure shell of a general cylindrical shape with its axis vertical and firmly secured to a substantially horizontal tubular plate, the lower face or inlet face forms a wall of a water box for supplying pressurized water forming the primary fluid to the steam generator.
This water box of a generally hemispherical shape is crossed by openings at which tubings are welded for connection to the hot branch and to the cold branch of a loop of the primary circuit of the nuclear reactor.
Nuclear reactors have to be periodically shut down for carrying out maintenance, replacement and repair operations, such as the handling of fuel assemblies.
This downtime gives the possibility of also carrying out maintenance of the steam generators.
In order to carry out certain maintenance or repair operations on the steam generators, during the downtimes of the nuclear reactor, it may be necessary to have operational staff intervene inside the water box.
The primary circuit and the vessel of the nuclear reactor are filled with water during the maintenance operations of the nuclear reactor so that it is necessary to empty the water box and then isolate it from the primary circuit so that operators may intervene inside this water box.
In order to allow intervention of operators inside the water box, devices for sealing tubings joining this water box to the primary circuit, which include stoppers also called sealing plugs, are set into place inside the tubings of the water box prior to the interventions.
In order to set this stopper into place, an attachment ring of the stopper is welded inside the water box around the opening of the tubing in its portion opening out inside the enclosure in a coaxial arrangement with respect to the tubing.
The stopper also includes a sealing member, intended to be applied onto the inner surface of the attachment ring and/or of the opening of the tubing.
It is generally required that the stopper be made in several portions which are assembled inside the water box or in a foldable form insofar that the stopper has to be introduced into the water box through a manhole, the diameter of which is generally smaller than the diameter of the opening of the tubing in which the stopper is placed.
This type of stopper should meet design requirements related to the conditions of use, notably in terms of pressure, temperature and dosimetry for the operators.
Indeed, the stopper should be able to withstand a pressure of the order of 5 bars on the side of the primary circuit and a temperature which may attain 120° C. in the case of loss of cooling of the reactor.
In order to achieve maximum reduction of the dosimetry integrated by the operators, during interventions, the stopper should be able to be installed and removed during a short human intervention and be preferably screwed by tele-operation onto the attachment ring.
For this purpose, a stopper also called a sealing plug is known, consisting of three adjacent portions which, once assembled form a basin including a first cylindrical portion closed by a bottom, the diameter of which is smaller than the inner diameter of the ring, and a second peripheral portion with a flat annular shape, the outer diameter of which is greater than the inner diameter of the ring.
In this case, the seal is ensured by two inflatable peripheral annular gaskets which are inserted between the inner surface of the ring and the external surface of the cylindrical portion of the stopper, when this stopper is set into place, as well as by a static gasket which will either bear against the inner surface of the opening of the tubing, or against the upper surface of the ring, and a peripheral retaining portion in the form of a rim which will be applied against the upper surface of the attachment ring.
Maintaining the stopper in position is generally ensured by attachment members, such as for example screws, which cross the peripheral portion of the stopper and which are screwed into threaded orifices made in the ring.
The main drawback of this kind of stopper lies in the fact that it requires the gasket to be connected to an inflating unit and that it requires continuous monitoring of the inflation of the gaskets guaranteeing the seal.
Further, it was found that this kind of stopper has mechanical strength and in particular, resistance to temperature or to pressure of a fluid being exerted on one side of said stopper, which may be insufficient for repeated uses and for long durations, or for accidental situations.